1. Field of the Invention
The present invention relates to a photomask, more particularly to a photomask suitable in photo lithography to form fine patterns.
2. Description of Related Art
Currently, in a manufacturing process of a semiconductor device, photo lithography technique is mainly used to form patterns for semiconductor devices on a semiconductor substrate. In photo lithography, a mask pattern formed on a photomask is transferred onto a photo resist film applied for the semiconductor substrate by using projection photolithography apparatus, and a photo resist patterns is obtained by developing the photo resist film afterwards.
A photomask employed in the photolithography process is a photolithography negative plate having a pattern consisting of a transparent region and light shielding region. In case of reduction rate other than 1:1, the negative plate is referred to as a reticle. But, it is referred here to as a photomask in both cases.
In recent years, high integration of semiconductor device is highly advanced, and semiconductor device patterns is set to be more and more fine almost coming to the vicinity of resolution limit of photolithography apparatus. In pattern formation in the vicinity of such resolution limit, influence of optical proximity effect (OPE) becomes outstanding. It is a phenomenon that size and geometry of transferred pattern varies with this optical proximity effect affected by another neighboring pattern. If the influence of this optical proximity effect becomes outstanding, it is difficult to transfer a mask pattern onto a photo resist film in semiconductor substrate in desired size and geometry. Reduction of yield and increase of reproduction rate of semiconductor device are caused accordingly.
In order to reduce the influence of optical proximity effect, optical proximity correction (hereinafter referred to as OPC) technique is employed for manufacture of a photomask. It is a method which enables to get a photo resist pattern of nearly desired size and geometry of designed pattern by deforming a mask pattern from a designed pattern formed on a photomask, in consideration with the influence of optical proximity effect. As such OPC methods, a mask biasing, and model base OPC are proposed. A mask biasing (refer to the following Patent Document 1) is the one to change the size of a mask pattern in order to obtain a desired dimension. A model base OPC is the one to arrange a fine pattern called hammer head or jog, which dimension is lower than the resolution limit, in order to obtain a desired shape.
[Patent Document 1]
Japanese Patent Application Publication No. 9-304913 (cf. paragraph 0014 in particular)
FIG. 4 is a chart which shows an effect by an OPC of mask biasing, in which a line and space (L/S) patterns of 110 nm width are formed by the photomask adopting a mask bias OPC. In the chart, pitch of L/S patterns is plotted in horizontal axis, and a finished size (critical dimension) of photo resist pattern is plotted in vertical axis. As comparison, finished size (no OPC) of photo resist pattern of the photomask which was formed without OPC was shown. From this chart, it is understood that fluctuation of finished size for L/S pattern pitch can be controlled as small, by using the photomask which is manufactured by applying mask biasing OPC.
However, in a mask bias method described in the above, there is a problem that mask size can be only changed by a fixed pitch of discontinuous value when a mask pattern size is corrected against a designed pattern size. All the mask data for making an photomask and the exposure data of drawing device reside on the grid of equal interval both in x and y directions. And this grid size is determined by restriction in mask drawing device.
Presently, it is possible to set about 1.0 nm in size on a wafer (semiconductor substrate) for the smallest grid size. However, if a grid size is lessened, then it causes a problem that mask drawing hour is increased. Thus, when productivity is considered, it is necessary to keep grid size in a certain dimension. Thus, it is difficult to perform dimensional control of high accuracy in forming advanced fine patterns.
In particular, when a fine pattern is formed, which is designed as fine coming to the vicinity of the resolution limit of photolithography apparatus, problem occurs that influence of so-called MEF (mask error factor) grows bigger. That is, dimensional change of mask pattern is amplified against a photo resist pattern, and it is transferred. MEF is a proportionality constant when size fluctuation of mask pattern is transferred onto a photo resist pattern. For the case MEF=1.0, size fluctuation of mask pattern is transferred by 1:1 onto photo resist pattern, and size fluctuation of mask pattern in case of MEF=2.0, is transferred by 1:2 onto a photo resist pattern.
FIG. 5 shows a simulation consequence of MEF in pattern of the hole diameter of 120 nmφ with the pitch of 200 nm. But, it is calculated under the assumption that mask pattern of mask size 130 nmφ is transferred to a finished size (critical dimension) of photo resist pattern, in consideration of margin security in a manufacturing process. In addition, in FIG. 5, cases are disclosed wherein lens numerical aperture NA of photolithography apparatus is 0.70 and 0.75.
Based on this FIG. 5, it was found as shown in the following table 1, that size fluctuation of mask pattern become about 6-9 times, and is transferred onto a photo resist pattern when the value of MEF is calculated in the range of mask size=130±4 nmφ which is mask biased with ±4.0 nm against the designed pattern.
TABLE 1CD [nm]Mask designNA126130134MEF0.786.2120160.59.30.7596120147.36.4
Usually, mask bias is symmetrically applied with regard to the center of the mask pattern, that is, symmetric with right and left, and up and down. Thus, as mentioned above, if OPC by mask biasing is performed at the smallest grid 1.0 nm of mask drawing, at least mask bias of 2.0 nm (or mask bias of 1.0 nm right and left, or up and down) is applied. Suppose MFE is 6.0 according to the above mentioned simulation, size fluctuation of photo resist pattern is 12.0 nm. Thus, in actual condition, it is not possible to perform the dimensional control of photo resist pattern with accuracy of less than or equal to 12.0 nm in OPC by mask biasing.
And, as same as the mask biasing described above, in model-base OPC arranging a fine pattern less than or equal to the resolution limit as against design pattern, similar problem occurs so that there is a constraint in configuration of fine pattern.